Cables

Right. Cables. Well, you should not need to add or subtract stitches when you make cables in the design, BUT you must keep your tension even. Another must, if you want well defined cables, is that there should be at least two stitches either side of each cable that are different from the cable and the border; i.e.

5 stitches garter. 2 st. reverse stocking st. 8 st. st.st.[cable] 2 st. rev st st. 5 st. gart.st.

I hope this helps, but if it's as clear as mud, I can demonstrate at our next meeting, which is next week. Also, remember that if you are doing a main ground of something like moss stitch, you need to have an odd number of stitches to get good border definition.
Have fun!